In the food industry and the chemical industry fields, for example, a type of evaporation device called “falling film evaporation device” is used to recover a solvent from a liquid containing foreign substances and impurities or to concentrate the liquid.
FIG. 16 is a diagram schematically showing an evaporation system including a conventional falling film evaporation device.
An evaporation system 900 includes a raw material tank 910 containing a raw material liquid serving as a raw material, a falling film evaporation device 800, a vacuum pump 920, and a condenser 930. The raw material liquid is caused to flow from the raw material tank 910 to a preheater 906 through a conduit 904 by driving of the pump 902, temporarily preheated in the preheater 906, and then fed to the evaporation device 800.
FIG. 17 is a diagram schematically showing a portion of a cross section of the evaporation device 800 constituting the evaporation system shown in FIG. 16.
As shown in FIG. 17, the evaporation device 800 includes an agitation vessel 810, a first rotating shaft 820 extending in the vertical direction within the agitation vessel 810 and being rotatable in the horizontal direction, a plurality of supports 822 individually extending in the horizontal direction from the first rotating shaft 820 in an upper portion and a lower portion of the agitation vessel 810, and rollers 826 extending downward from the respective supports 822 and being provided so as to come into contact with an inner wall of the agitation vessel 810. An outer circumference of the first rotating shaft 820 is covered by a second rotating shaft 821. The first rotating shaft 820 and the second rotating shaft 821 are independently rotatably connected to a driving motor portion 840. Moreover, in many other conventional evaporation devices, the first rotating shaft 820 and the second rotating shaft 821 are integrally configured and rotate at the same rotation rate.
The raw material liquid 834 fed from the raw material tank is supplied to an upper portion of the inner wall of the agitation vessel 810 through supply ports 832 extending in the horizontal direction from the second rotating shaft 821, while being rotated as per driving of the driving motor portion 840. After that, the raw material liquid 834 flows downward along the inner wall of the agitation vessel 810 while forming a wet surface. On the other hand, an outer circumference of the agitation vessel 810 is covered by a jacket 812 that can be heated by steam, for example, and, as a result of being heated via the jacket 812, a volatile component contained in the raw material liquid evaporates while the raw material liquid flows down. The evaporated volatile component is fed to the condenser 930 (FIG. 16) provided outside the evaporation device 800 through a vapor outlet 860. The volatile component is cooled in the condenser 930 and then collected as a distillate liquid. On the other hand, in FIG. 17, components contained in the raw material liquid other than the above-described volatile component flow down the inner wall of the agitation vessel 810 as is, and are discharged to the outside of the evaporation device 800 through discharge ports 880 provided in a bottom of the agitation vessel 810.
While the raw material liquid flows down within the agitation vessel 810 as described above, the driving motor portion 840 drives the rollers 826 provided on the respective supports 822 to circle along the inner wall of the agitation vessel 810 while being in contact therewith.
FIG. 18 is a diagram schematically showing a cross section of the conventional evaporation device 800 shown in FIG. 17 taken in the direction A-A′. In the evaporation device 800, the rollers 826 are in contact with and circle along the inner wall of the agitation vessel 810 heated by the jacket 812, thereby forcibly subjecting the raw material liquid present on a heat transfer surface of the inner wall to surface renewal, and thus, the evaporation efficiency can be increased. Although FIG. 17 shows that the rollers 826 are provided, in conventional evaporation devices, wipers may be provided instead of the rollers 826.
However, some matters of concern have been pointed out with respect to such evaporation devices.
One of those matters is that the supplied raw material liquid passes down the inner wall (heat transfer surface) within the agitation vessel by flowing down only once through a so-called “one path”. Also, in the case where the raw material liquid contains a large amount of a volatile component or in the case where the volatile component cannot be sufficiently evaporated while the raw material liquid flows down the inner wall, it is considered that the remaining components is discharged through the discharge ports 980 as is. For this reason, it has been recognized that the use of such an evaporation device for a raw material liquid that is required to be sufficiently concentrated is difficult.
Another matter is that rollers such as those shown in FIG. 17 or wipers are continuously in contact with the heat transfer surface and are thus likely to wear out. For this reason, regular replacement is required, and it has been pointed out that the working hours, labor, and costs for maintenance increase accordingly.
Another matter is that, in the case where the evaporation device is to be stopped, since the temperature of the inner wall is higher than the liquid temperature, if the supply of the raw material liquid is stopped as is, the rollers or the wipers, which are in contact with the inner wall, will deform or deteriorate due to high temperature. For this reason, when the evaporation device is to be stopped, it is necessary to continue the supply of the raw material liquid until the temperature of the inner wall falls.